The British Medical Somethingorother did a study on this a while back. The carbon monoxide levels & general crud levels in the blood of the tin-box people is a fair bit higher than that of the cyclist, despite them being 'protected' by their box, cos the regular cyclist's cardio vascular systems are more efficient and, as Seanholio sez, they're sucking each other's fumes while we're above it all <smug>

Believe it or not, as a cyclist you're actually in total control of how heavily you're breathing at any particular moment. If you think you're sucking in too many car fumes, try backing off the intensity of the ride by about 20%. You'll probably find yourself breathing much easier, as it's only really that last 20% of speed that takes most of the effort (the old 80/20 rule again).

Something else you can do, breathe through the nose rather than gulping it down through the mouth. The human nose is designed to filter out some of the particles. It won't get all of them, or even most of them, but it's something.

However, there really isn't any need to worry. There have been a lot of studies done on this, and all conclude that the fumes car drivers breathe are a lot worse than what the cyclist gets. Remember that most drivers have their windows wound up and the air conditioning on all the time. This effectively means that when the fumes enter the car, they have nowhere to go. On a bike at least you've got the wind to disperse them a little bit.

It's also another reason to pick your way through to the front of the queue at red lights. The air quality at the front is noticeably better than at the back.